Aug 25 (Reuters) - A drop in tech stocks weighed on the Nasdaq on Friday and erased some gains in the S&P and Dow that came after Federal Reserve Chair Janet Yellen stayed mum on monetary policy and a key White House official said tax reforms were on the horizon.

Yellen, in her speech at the annual meeting of central bankers in Jackson Hole, Wyoming, stuck to the theme of financial stability.

She said the reforms put in place after the 2007-2009 financial crisis have strengthened the financial system, without impeding economic growth.

“There was a risk that Yellen would’ve said something (on monetary policy). She didn’t say anything, so people can do what they wanted to do in the first place, which was position themselves for September,” said Marc Chandler, chief global currency strategist at Brown Brothers Harriman.

European Central Bank head Mario Draghi is scheduled to speak in the afternoon and investors will look for any comments on the central bank’s tapering plans.

At 12:42 p.m. ET (1642 GMT), the Dow Jones Industrial Average was up 55.26 points, or 0.25 percent, at 21,838.66 and the S&P 500 was up 6.91 points, or 0.28 percent, at 2,445.88.

The Nasdaq Composite was down 4.28 points, or 0.07 percent, at 6,267.05.

However, traders said volumes were low, as they tend to be during the summer months, making market moves more pronounced.

U.S. stocks got off to a strong start after President Donald Trump’s chief economic adviser, Gary Cohn, said the White House would turn its attention to the long-awaited tax reform agenda from next week.

Cohn, in an interview with the Financial Times, also said that despite pressure on him to both resign from and stay in his post, he was “reluctant to leave”.

In the past two weeks, speculation about Cohn’s departure, along with the recent unrest in the White House, had sparked concerns about the Trump administration’s ability to implement its pro-growth agenda, including on tax reform.

The technology index was off 0.03 percent, making it the only one among the 11 major S&P sectors in the red.

Broadcom’s 3.5 percent drop after its quarterly earnings report weighed the most on the on the tech index as well as the Nasdaq and S&P.

Oil companies including Chevron and Schlumberger rose more than 1 percent, helped by a rise in oil prices as Hurricane Harvey drew closer to the mainland.

Shares of Autodesk were up about 4 percent after the software maker raised its forecast.